PROJECT SUMMARY/ABSTRACT Abdominal aortic aneurysm (AAA) is the 13th leading cause of death in the United States for men of all races aged over 55. Except for invasive surgical interventions, no other treatment strategy is available to inhibit the growth or rupture of established AAA. We have been studying immunopathogenesis of AAA to develop a non-surgical treatment strategy. Our reports demonstrate that during AAA formation, neutrophils and B cell infiltrate aorta and depletion of either of the cell types attenuates vascular inflammation, retains aortic morphology and protects mice from AAA formation. Neutrophils secrete B cell activating factor (BAFF) which can activate B cells to produce antibodies. Our preliminary study demonstrates that genetic or pharmacological depletion of BAFF reduces the number of B2 cells, attenuates AAA formation, and decreases plasma level of only two of the antibodies IgG1 and IgG2, which are known to be the pathogenic in atherosclerosis. BAFF is secreted as a 3mer, which can bind to the BAFF receptor BR3. Interestingly, the 3mer can associate to a 60mer which can bind to three of the BAFF receptors BR3, TACI, and BCMA. Our cell signaling, RNA sequencing and metabolism studies suggest that the 60mer, but not the 3mer, activates B cells and promotes metabolic reprogramming, which are attenuated by inhibition of NF-kB signaling. We have developed an antibody (anti-DE Ab) against the association site of BAFF 3mer and found that the anti-DE3 Ab suppresses B cell activation in neutrophil-B cell co-cultures and in mice. Taking these findings together, we hypothesize that BAFF 60mer, not the 3mer promotes AAA formation and inhibition of 60mer formation will attenuate AAA growth. To test this hypothesis, we will elucidate the critical role of BAFF in neutrophil-B cell inflammatory crosstalk during AAA formation in Aim 1 and examine if inhibition of BAFF 60mer formation attenuates neutrophil-B cell inflammatory crosstalk, B2 cell activation and AAA formation in Aim 2. Our approach involves multiple disciplines. Surgical methods will be used to induce AAA in mice. The role of BAFF 3mer and 60mer in AAA formation will be determined by injecting recombinant BAFF 3mer and 60mer to BAFF-/- mice. Single cell RNA-Seq, cellular signaling, imaging flow cytometry, and Seahorse extracellular flux assays will be used to determine activation of B cells isolated from murine AAAs. Involvement of BAFF receptors in BAFF-mediated AAA formation will be determined by using BR3-/-, TACI-/- and BCMA-/- mice or anti-BR3, anti-TACI and anti- BCMA antibodies. Finally, we will develop novel monoclonal anti-DE3 Abs to inhibit BAFF 60mer formation and validate using biochemical and biophysical approaches and injecting to mice. We will compare the efficacy of anti-DE Abs with anti-BAFF biologics Belimumab and Atacicept used for treatment of lupus. Thus, this proposal will accomplish part of our major goal in identifying BAFF 60mer, not the single BAFF molecule as a therapeutic target in vascular diseases. Furthermore, our anti-DE Ab does not deplete B cells and hence, is advantageous over available anti-BAFF biologics.